How to Fully Japanese-Localize Ubuntu: Complete Guide to Language, IME, Fonts, and Locale

1. Benefits and Prerequisites of Japanese Localization in Ubuntu

The Goal of Localization — “Not Everything Becomes Japanese at Once”

“Japanese localization” in Ubuntu is not a single, unified process. In reality, it consists of multiple independent layers, and only when all of them are properly configured does the system feel truly localized.

• UI language (menus and dialogs): Display language of the desktop environment and settings
• Regional formats: Date, currency, decimal separators, and week start rules
• IME (Japanese input, e.g., Mozc): Input foundation for Hiragana and Kanji conversion
• Fonts (Noto CJK / IPA): Readability, glyph width, and diacritic clarity
• Application-specific language packs (e.g., LibreOffice): Some apps require additional packages
• Locale (LANG/LC_*): Character encoding and message language for terminals and some applications

Because of this structure, it is common for parts of the system to remain in English even after selecting Japanese during installation. This article explains a GUI-first approach to quickly improve usability, followed by package installation and fine-tuning to achieve a practically “complete” Japanese environment.

Benefits of Japanese Localization

• Improved productivity: Settings and error messages are easier to understand, enabling faster troubleshooting.
• Consistent formatting: Dates, numbers, and currency follow Japanese conventions, reducing misunderstandings.
• Better readability and appearance: Proper Japanese fonts eliminate blurred glyphs and awkward spacing.
• Lower learning cost: Documentation and help can be read in Japanese.

Required Time and Knowledge Level

• Estimated time: About 10 minutes for GUI settings only; around 30–40 minutes including packages and fonts.
• Required knowledge: Basic settings navigation and a few terminal commands (copy & paste is sufficient).
• Re-login / reboot: Re-login is required for language and IME changes; sometimes a reboot is necessary.

Recommended Preparations

• Internet connection: Required to download language packs, fonts, and IME.
• System updates: Keeping packages up to date ensures smooth installation.
• Administrator privileges (sudo): Needed to install additional packages.

Why Some Parts Remain in English — and How to Fix Them

• Distribution differences: Snap or Flatpak apps may manage language resources separately.
• Application-specific language packs: LibreOffice, for example, requires an additional -l10n-ja package.
• Unconfigured locale: Terminal messages remain English → fix by properly configuring locale.
• IME not integrated: Mozc not added to input sources or re-login not performed.

Article Roadmap

1. GUI-based localization (fastest visible improvement)
2. Language packs and IME installation (language-pack-ja, ibus-mozc)
3. Font optimization (Noto CJK)
4. Application-specific localization
5. Pitfalls and checklist

Start with GUI settings, then configure IME and fonts for usability, and finally adjust applications and locale. This order is the most reliable and least confusing.

2. Apply Japanese Settings via GUI

Why You Should Localize the Interface First

Changing the GUI language alone significantly improves daily usability. This is the fastest way to feel the impact of localization and also serves as a foundation for later IME and font configuration.

Steps to Set Japanese via Settings

On Ubuntu Desktop, follow these steps:

1. Open Settings from the dock
2. Select Region & Language
3. Under Language, choose Japanese and click Install
4. Set Formats to Japan
5. Sign out and log in again

Menus and system labels will now appear in Japanese.

(Important) Why “Formats” Must Also Be Set to Japan

Even when the language is Japanese, formats are often left as US or other regions, causing issues such as:

• Decimal separator differences
• Date format remaining MM/DD/YYYY
• Currency symbol staying as $

Aligning both Language and Formats to Japan ensures consistent behavior, which is especially important for technical work and numerical data.

Re-login vs Reboot Guidelines

• Language pack changes → Re-login is usually sufficient
• IME or application messages → Reboot may be required

3. Installing Additional Language Packs (Ubuntu Standard Packages)

Why GUI Settings Alone Are Not Enough

Even after setting Japanese via the GUI, some internal components may still operate with English messages.
This is because Ubuntu distributes language components in separate modules.
In other words, only by installing additional language packs can the internal language be fully aligned.

Update Package Information First

Start by updating the repository information.

sudo apt update

Because Ubuntu packages are frequently updated,
skipping this step may result in missing language packs or installing outdated versions.

Installing Japanese Language Packages

The following two packages are essential for Japanese localization:

• language-pack-ja
• language-pack-gnome-ja (effectively required for GNOME users)

sudo apt install language-pack-ja language-pack-gnome-ja

* GNOME is the default desktop environment for Ubuntu

* For KDE or other environments, packages such as -kde-ja may be applicable

What to Check After Installation

After installation, Ubuntu switches its internal message catalogs to Japanese.
A re-login is required for the changes to take effect.

You can verify the settings with the following command:

locale

Example output (ideal state):

LANG=ja_JP.UTF-8
LC_CTYPE="ja_JP.UTF-8"
LC_TIME="ja_JP.UTF-8"
...

If any entries still show en_US.UTF-8,
they can be corrected later by adjusting IME, fonts, and locale.

Goal of This Step

• Align the OS internal message language to Japanese
• Ensure consistency between GUI appearance and command-line output

4. IME (Japanese Input: Mozc) Configuration

The Input Method Defines the “Real” Localization Experience

Even if the UI is in Japanese, it is not practical unless you can actually input Japanese text.
On Ubuntu, Mozc, based on Google Japanese Input, offers excellent stability and conversion accuracy and is the standard choice.

This section ensures a reliable environment where Hiragana and Kanji input works correctly.

1) Install Mozc

Run the following command in the terminal:

sudo apt install ibus-mozc

This single command installs Mozc and integrates it with IBus, the input framework.

Ubuntu Desktop uses IBus by default.

KDE environments using Fcitx require different packages.

(This guide assumes the Ubuntu Desktop default.)

2) Add Mozc to Input Sources

1. Open Settings
2. Go to Region & Language
3. Click “+” under Input Sources
4. Select Japanese → Mozc Japanese Input
5. Place it below the US keyboard (commonly preferred)

You can now switch to Japanese input.

3) Re-login Is Often Required

A common issue is that Kanji conversion does not work immediately after adding Mozc.
Because IMEs run within the user session, re-logging in is the most reliable solution.

4) How to Confirm Input Is Working

In a text editor or browser address bar, try switching input using:

• The Half-width/Full-width key
• Super + Space (depending on environment)

If the indicator toggles between “A” and “あ”, input is working correctly.

5) Why Mozc Is Recommended

→ Enables immediate, practical Japanese input.

5. Optimizing Japanese Fonts

Fonts Affect Both Appearance and Work Speed

When using Ubuntu in its default state,
you may notice awkward spacing or blurred diacritics.

This is not simply a matter of preference.
It happens because Japanese text is rendered without proper font optimization.
CJK fonts sometimes do not perfectly match Japanese glyph width or weight.

Recommended Font: Noto CJK

Developed jointly by Google and Adobe, Noto CJK works exceptionally well with Ubuntu.

Installation is simple:

sudo apt install fonts-noto-cjk

This alone significantly improves Japanese text rendering across the system.

6. How to Handle Cases Where Some Parts Remain in English

Common Scenario: Everything Is Japanese Except “That One App”

Even after the GUI is localized and Mozc works correctly,
it is not uncommon for only certain applications to remain in English.

This does not mean your configuration is wrong.
It happens because applications may be managed using different distribution methods or separate language packs.

Common Cause 1: Snap / Flatpak Applications

Ubuntu increasingly distributes applications as Snap packages.

Snap packages are containerized and often include their own language resources.
As a result, OS-level localization and application language settings are handled separately.

→ In such cases, OS localization alone does not affect the app’s UI language.

Recommended Actions

• Stop using the Snap version and switch to the deb version
• If available, switch to a Flatpak version that includes Japanese resources

For applications like VS Code or Firefox,
simply switching from Snap to deb often resolves Japanese localization issues.

Common Cause 2: Applications with Separate Language Packs

Typical example: LibreOffice

LibreOffice requires an additional package for Japanese localization:

sudo apt install libreoffice-l10n-ja

Installing this package replaces most of the UI with Japanese.

Common Cause 3: Locale Settings Are Not Fully Aligned

If the GUI is Japanese but terminal messages remain in English,
it usually indicates that locale settings are inconsistent.

locale

If the output is not ja_JP.UTF-8,
adjustments are required in the next steps.

Decision Guide

“Mostly Japanese but partly English” is a normal and expected state.

Resolving these cases brings you one step closer to a fully localized system.

7. Common Pitfalls and How to Avoid Them

1) Skipping Re-login or Reboot

Language packs and IMEs operate within the user session.
Installing them alone may not fully apply changes.

Guideline:

“Mozc does not work → forgot to re-login” is extremely common.

2) Snap Versions of Firefox / VS Code Manage Language Separately

Firefox (Snap by default since 2023)
VS Code (Snap when installed via Ubuntu Software)

These applications often handle localization independently from the OS.

Improvement examples:

• Firefox → deb version
• VS Code → official Microsoft .deb package

Snap itself is not inherently bad,
but deb packages are usually faster for Japanese UI consistency.

3) Locale Settings Are Incorrect

GUI is Japanese but terminal error messages remain English — a classic symptom.

Check with:

locale

Example:

LANG=ja_JP.UTF-8

If not set,
reconfigure locale as described later.

4) Fonts Not Installed, Resulting in Awkward Appearance

If Japanese text looks odd,
it is often because Noto CJK is not installed.

5) Language Is Japanese but Regional Formats Are Still US

If Formats are not set to Japan,

• Dates
• Decimal separators
• Currency

will follow non-Japanese rules.

Always align both Language and Formats to Japan.

8. Summary

Japanese localization in Ubuntu is not a one-step process.

UI → Language Packs → IME → Fonts → Application-specific settings → Locale

These layers must be configured in order.

However, by focusing on just three key points:

• Do not skip re-login
• Handle Snap applications separately
• Use Noto CJK fonts

Most Japanese localization issues disappear.

The entire process takes about 30 minutes once familiar.
With a little tuning, Ubuntu becomes a highly comfortable environment for working and creating in Japanese.

9. FAQ

Q. The GUI is Japanese, but terminal messages are still in English.

A. The locale is likely not unified.
Check whether locale shows ja_JP.UTF-8.

Q. Mozc is installed, but Japanese input does not work.

A. Make sure Mozc is added to Input Sources.
If it is, re-login.

Q. Only LibreOffice remains in English.

A. Install libreoffice-l10n-ja.

Q. Are Japanese fonts mandatory?

A. Not mandatory, but highly effective.
They significantly improve clarity and readability.

Q. Are Snap applications harder to localize?

A. They often are.
Switching to deb versions usually resolves the issue faster.