OLED localization guide (#390)

docs/developers/Firmware/oled-l10n-guide.mdx

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+---
+id: oled-l10n-guide
+title: OLED localization guide
+sidebar_label: OLED localization
+---
+
+# OLED localization guide
+
+## 1. Create an extended ASCII custom font
+Use a glyph editor to create a new font file. The easiest way is to use the online [glyph editor][glyphEditor] from the OLED library. [Glyph editor source code][glyphEditorSource]. Copy and paste the existing font, modify it according desired codepage and save the new font file in `graphics/font` folder.
+
+Please note that the used font file format differs from common Adafruit GFX.
+
+## 2. Update the `customFontTableLookup` function in `Screen.h`
+To map the double-byte UTF-8 code to the corresponding extended ASCII character of the desired codepage update the `customFontTableLookup` function in the `Screen.h` file. You need to modify the `switch (last)` statement: use left byte from UTF-8 code in the `case` label to map charachter's right byte to its extended ASCII code by specifying an offset.
+
+## 3. Define language and font in `Screen.cpp`
+```
+#ifdef OLED_{LANG_NAME}
+#include "fonts/OLEDDisplayFonts{LANG_NAME}.h"
+#endif
+
+...
+
+#ifdef OLED_{LANG_NAME}
+#define FONT_SMALL ArialMT_Plain_10_{LANG_NAME}
+#else
+#define FONT_SMALL ArialMT_Plain_10
+#endif
+```
+
+## 4. Define language in `variant/*/platformio.ini`
+```
+build_flags =
+  ${esp32_base.build_flags}
+  -D xxxxx
+  -D OLED_{LANG_NAME}
+  -I variants/xxxxx
+```
+
+   [glyphEditor]: <https://rawgit.com/ThingPulse/esp8266-oled-ssd1306/master/resources/glyphEditor.html>
+   [glyphEditorSource]: <https://github.com/ThingPulse/esp8266-oled-ssd1306/tree/master/resources>